I want to modify my (old) forks.

Can I just do a drawing and get a company to machine it for me, as a one-off? If so, is it going to cost me £100s?

Anyone on here have a lathe and want to do it for me? 🙂

Yes, just provide a drawing to a local engineering fabrication firm and they will sort you out.
If you aren’t fully clued up on tolerancing etc. it would probably be worth speaking to them about it so they understand what you are going to achieve.
They can probably also advise on how to draw something up that minimises cost.

Obviously the tighter tolerances are the the more expensive the part will be.
Do you know what material you want to use?

If you know someone that works that works at an engineering firm this is best as loads of places slow a little homework out of hours that someone will do for a bit of cash over lunch.

Ask around your mates as someone may not be a machinest but may work with some.

If you are based anywhere near Bournemouth i could maybe help depending on details as we have a full machine shop at work.

This is the air spring – it sits inside that large bronze coloured cylinder, but in this case the bottom has been popped out so you can see:

What I want to do is replace that internal shaft to one with holes in different places. Normally the end cap (the bit with the air valve in it) is bonded (I think) to the shaft, but I happen to have an end cap lying around that has no shaft.

So I would need a shaft of the right external diameter, and then a piece made to fit into the end cap with a flange to accept the end of the shaft, which I’d then glue into place in the end cap. I then drill my own air hole in the end of the shaft. This allows me to bin that floating piston thing and then pressurise the negative chamber by means of the valve. Currently it’s a self-setting thing by means of that notch you can see in the shaft. And it’s not enough -ve air I think.

You’re probably talking 3 figures for a one-off unless you can get ‘mates rates’ from someone who can access a workshop and you’re only paying for materials. Assuming the shaft is hollow makes it quite tricky and might require specialist tooling.

Well I’d imagined starting off with a piece of tube that’s already a tube, and simply turning it down to reduce the outside diameter. Internal diameter doesn’t matter. The air hole I mention is just a hole in the side of the thing, which I’d do with my own pillar drill.

Not sure exactly which bit you want, but that shaft looks to have a very finely machined and hard anodised finish – not just a bit of ally bar chucked in a lathe. And as dovebiker suggests long / deep small diameter accurate holes are a specialist thing (not always made with a drill).

Have you found any suitable raw tube listed anywhere?

Is there going to be a sliding seal surface on your new shaft (which is the bit requiring the fancy surface finish)?

Depends if you can start with a tube that’s a standard size. It could be non standard. If you can measure the tube and check on line to see if it’s a standard size. If it’s not you’re looking at making a tube from rod.
I would guess it is a standard size, but worth checking.

But the only accurate bits are external diameter – the internal dia and the extra hole in the end don’t matter. Surely it’s just a question of taking an off the shelf tube and turning it down?

The external finish is the bit that I would imagine would be hard. Possible to either not bother and just have a rough finish, or anodise it myself.

Of course if it’s a standard tube size I’d just have to have a piece made to fit it to the end cap.

not cheaper to buy new to you forks?

Hard anodised aluminium bar / tube. Good luck 🙂

If you ask nicely they might send a sample.

https://www.igus.com/wpck/16405/drylin_r_awmp

I’m curious as to why you’re doing this? Is something broken?

Surely it’s just a question of taking an off the shelf tube and turning it down?

So long as the wall thickness is large enough if using a standard length of tube.

i was thinking some Igus tube too.

As that has to run in a seal with low drag the surface finish and tolerance are critical and that is what is going to cost you ££££

Igus are pretty good with free samples if you’re after bearings, not so sure about the metal bits. Price details should be on the web site.

Is that a windy-down marz 66 I see before me? Thought I had the last of those

I’m curious as to why you’re doing this? Is something broken?

No, just not quite right still. The -ve pressure isn’t adjustable, and I want to add more. The mod will work if I can make it.

If I can buy a bit of tube and have a thing machined for a reasonable number of pounds then it’s worth a shot. And it’d be a satisfying project.

Is that a windy-down marz 66 I see before me? Thought I had the last of those

Mine doesn’t wind down, I had that fixed under recall. It’s always annoyed me though – loads of stiction. So I’m thinking about this mod, also going to see if I can replace the double Marz seal with a single Rockshox one and foam ring. Maybe also Fox Green oil …